1. Field of the Invention
The present invention generally relates to porous polymeric materials, particularly useful for polishing in the semiconductor industry. More particularly, materials of the present invention are formed by an electrophoretic deposition technique. (As used in this application the term "polishing", or any form thereof, is intended to include planarizing and any corresponding form.)
2. Discussion of Related Art
The requirements of extremely high circuit density in semiconductor devices have created the need for high precision surfaces. The need for these ultra smooth and planar surfaces requires extremely uniform and homogeneous polishing pads. Accurate delineation of semiconductor manufacturing processes requires little or no variation from pad to pad.
Traditionally, polishing has been accomplished by introducing an abrasive polishing fluid (slurry) between a polishing pad and a workpiece to be polished and creating relative motion between them. A widely used material for polishing pads is taken from a class of materials known as poromerics. Poromerics are textile-like materials that usually contain a urethane-based impregnation or coating having a multitude of pores or cells. The porosity facilitates the flow and distribution of the slurry.
Many poromeric materials used for polishing are similar to the material described in U.S. Pat. No. 3,284,274. These polishing poromerics are somewhat different from most other poromerics in that the surface of polishing poromerics contains large macropores or cells. It is believed that these large macropores or cells act to hold slurry and thus aid the polishing process. U.S. Pat. No. 3,504,457 describes the use of these materials in polishing silicon semiconductor substrates.
One prior art method for manufacturing poromeric polishing materials is to coat a solution of polymer onto a substrate and then immerse the coated substrate into a bath that will cause coagulation of the polymer. Once the polymer has been sufficiently coagulated, the remaining solvent is leached out and the product is dried. The next step in the manufacturing process is to remove the top skin by passing the material under a blade or under a rotating abrasive cylinder. Once the top skin is removed the underlying pores are exposed and open to the surface.
An important characteristic of this prior art product is the configuration of its cell structure. Because of the nature of the coagulation process, the pores and cells tend to increase in diameter as they penetrate deeper into the material. This means that the diameters of the cells exposed on the working surface of the material are relatively small compared to the underlying cell diameters. This can be readily seen on FIG. 3, which is a photomicrograph of a cross section of a Politex.RTM. (polishing pad manufactured by Rodel, Inc., Newark, Del.
In the polishing process there are disadvantages to the small surface opening of the pores. First, slurry can build up in the pores more readily than if the pore had larger surface diameters. The trapped slurry reduces the pad's polishing ability. Second, small pore openings translate to more pad surface area contacting the workpiece. For traditional polishing, this decreases the presentation of new slurry to the workpiece.
U.S. Pat. No. 4,841,680 describes a poromeric polishing pad having a working surface comprised of a microporous polymeric material which contains open cells that have their largest opening at the work surface and are deep enough to carry a relatively large quantity of slurry. The pad is made by conventional solvent/nonsolvent polymer coagulation technology. However, instead of removing the top skin, a second substrate is affixed to the top of the poromeric layer, and the first substrate is removed from the bottom to expose the pores at their largest diameter.
These prior art poromeric material manufacturing techniques introduce variation into the pad fabrication process at a number of steps. With the need for well defined semiconductor device manufacturing parameters, highly reproducible polishing results are extremely important. Therefore a need exists for a more uniform and homogeneous porous polymeric material suitable for polishing pads and other applications.
Electrophoretic deposition has been used to manufacture nonporous grinding materials. The process, such as that described in the article Development of Highly Homogeneous Pellets Applying Electrophoretic Deposition of Ultrafine Abrasives for Nanometer Grinding by Jinuichi Ikeno, Yasuhiro Tani and Hisayosi Sato (Annals of the CIRP Vol. 43/1/1994, pp 319-322), consists of mixing colloidal silica with polyvinyl alcohol solution, forming a deposition layer from the mixture using electrophoretic deposition, fabricating pellets by cutting the layer, and then drying the pellets.